1. Field of the Invention
The present invention relates to improvements in solid-state laser technology, and more specifically, it relates to means for controlling and eliminating parasitic oscillations in solid-state lasers.
2. Description of Related Art
Parasitics reduce the efficiency of a solid-state laser system by establishing undesirable and uncontrolled paths of laser oscillations that extract energy from the system. They are a consequence of internal reflections at surfaces of a laser medium at angles of incidence that are not intended for laser oscillation by the designer. If these stray rays can close their path and reflect into themselves, parasitic lasing will occur as soon as the gain in the laser medium exceeds the reflective losses (Koechner, Solid-state Laser Engineering).
There are a number of methods in the prior art that are all partially successful in reducing parasitic oscillations. Internal reflections can be avoided by roughening surfaces. If, however, pump light is to be introduced through a roughened surface, the pump efficiency is greatly reduced by scattering of pump light. This solution to the problem of parasitics appears to be most effective for flashlamp pumped systems where efficiency is not an important consideration, while diode laser pumped media would usually be pumped through polished surfaces.
Wedged or canted surfaces on rods and slabs can be designed to reduce parasitics due to internal reflections on polished surfaces. While this method provides a partially satisfactory solution, it rarely eliminates parasitics completely because it is difficult to design angled surfaces that will not allow any stray laser radiation to find a closed path within the lasing medium.
Rods with grooves have been successfully employed to drastically reduce parasitics. Especially whispering gallery modes that propagate circumferentially can be essentially eliminated at the expense of reduced efficiency when side pumping with laser diode bars.
Rods and slabs with undoped ends of a variety of flange designs have provided some protection from parasitics because they will reduce the number of possible closed paths and also reduce the gain of longitudinal modes.
Disk or plate amplifiers are designed for high average power operation, and suppression of parasitics has been the subject of numerous investigations. Bulk and surface parasitic modes have been identified as principal mechanisms of siphoning energy away from the intended laser action. The most common and most effective means of controlling parasitics occurring in the bulk of the medium is by attaching absorptive edge claddings to amplifier plates. This approach suffers from the thermal non-uniformities and stress that are generated by the heat of absorption in the cladding. Claddings tend to delaminate from the lasing medium, resulting in catastrophic systems failure. Another important disadvantage is that it usually is not possible to optically pump through the cladding layer because it often also absorbs pump radiation. In that case, amplifier disks have to be face pumped. Surface modes of single-pass amplified spontaneous emission are enabled by the higher gain coefficient at the surface due to pump light absorption, and represent the principal loss mechanism of large, edge-clad disks.
A means to reduce the gain of parasitic oscillations in laser slabs is to affix laser-inactive host medium to, e.g., the sides and/or end faces. This technique is partially successful, especially when it is combined with wedged surfaces, but will also not be able to prevent parasitics at high power densities since it is virtually impossible to prevent all closed paths of stray laser radiation. Similarly, antireflective coatings have a beneficial effect in reducing total internal reflections but again not at all angles of incidence.
A special case of a disk-type lasing medium where parasitics are difficult to suppress with the prior art are planar waveguide geometries that suffer from total internal reflections when their side surfaces are polished. Wedging the side surfaces is only partially effective in suppressing closed-path parasitics since, by the very nature of the waveguide design, more stray rays are reflected back into the lasing core layer than if the refractive index of the lasing and cladding layers were closely matched. Using absorptive side claddings such as dopants of Sm3+, Cr3+, Cr4+ and Co2+ in YAG, for lasing planar waveguides of Yb or Nd:YAG/YAG/sapphire usually is not possible when side pumping these structures because the pump light would also be absorbed by the absorptive layer.
Another method of avoiding parasitics claims that generating absorbing regions inside of a gain medium by spatially sectioning it into pumped and unpumped regions (U.S. Pat. No. 5,926,494, titled xe2x80x9cLaser Systems With Improved Performance And Reduced Parasitics And Methodxe2x80x9d by D. M. Pepper). While this approach is claimed to be applicable to systems with 3-level lasing ions and to be scalable to high powers, it again appears to suffer from absorption in some parts of the lasing medium and limited applicability.
None of the teachings of the prior art enable the elimination of parasitics by a simple and generally applicable method that still allows side pumping through polished surfaces without any absorption or thermal effects. The present invention overcomes the limitations of the prior art without being complicated or requiring the production of expensive structures.
It is an object of the present invention to reduce or eliminate parasitic oscillations in all known embodiments of solid-state laser amplifiers and oscillators.
Other objects will be apparent to those skilled in the art based on the teachings herein.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which consists of applying a pattern of grooves onto a polished surface where the spacing between grooves is large enough to allow the pump light from individual diodes of a diode bar to pump the lasing medium without any impediment such as absorptive cladding layers or a roughened surface. The depth of the grooves may vary from of the order of microns only, up to centimeters, depending on the laser architecture and the designed power output The polished surface may have an antireflective coating deposited for even better pump efficiency.
The present invention is practiced by providing a pattern of grooves that have depths and spacings between grooves so that total internal reflections are substantially eliminated. These grooves usually are perpendicular to the polished surfaces where TIR is to be frustrated but, depending on the specific laser architecture, grooves may form angles other than 90xc2x0 with these surfaces.
Another embodiment of the present invention provides undoped cladding layers of a thickness that corresponds to at least the depth of the groove required to frustrate total internal reflections of a lasing medium of given refractive index. In this way, the volume of the lasing medium is not affected by the groove pattern.
An embodiment of the invention was designed around a DILAS manufactured laser diode array. The width of the diode array was measured. The laser diode array emitter and isolation spacings are provided. In the diode array used in this example, there are 25 emitters. The cross sectional outputs of two Dilas laser diode arrays are shown. The output does appear to be uniform across the width of the diode. A design is provided that will control parasitic modes for a laser diode array having the Dilas emitter and isolation spacing. This example illustrates the design considerations applicable to other laser configurations. Examples of other laser mediums to which this design is directly applicable include a zig-zag slab laser with undoped side claddings and a diode-pumped rod laser.
An embodiment is described of an Nd:YAG rod that includes evenly spaced grooves that run the length of the rod. Alternately, the grooves may encircle the rod perpendicular to the length of the rod. Still another embodiment may include a crosshatched groove pattern to prevent parasitic oscillations from arising longitudinally and transversely. The grooves are designed to have a spacing that will allow a maximum reflected angle that is less than the critical angle. The grooves are shown as having a rectangular shape; however, other shapes such as v-shaped grooves and cylindrical grooves are usable in this configuration as well. The sides of the grooves may be made diffuse to further prevent parasitic oscillations. A higher refractive index cladding layer may be applied between the grooves to trap stray radiation. A pattern of absorbing species may be in between the grooves, either within the lasing medium itself, or in a cladding that is located on the area adjacent to the groove pattern.
A description is provided of an embodiment of the present invention that utilizes a generally tapered rod having a doped central rod portion 90 and undoped end caps. The use of undoped, flanged endcaps has proven useful in limiting the maximum length of the ray path that can be trapped in the laser rod.
An embodiment of a laser diode pumped zig-zag slab laser is described. The slab includes the groove architecture of the present invention and may further include grooves that traverse the slab perpendicular to its length. The present invention is also embodied in a microchip laser. The microchip laser includes the groove architecture of the present invention and may further include grooves that traverse the slab perpendicular to its length.